nullbios: credits management for 2 players

Ref dciabrin/ngdevkit-examples#7

nullbios/biosstart.S

@@ -127,6 +127,11 @@ RESET:
         /* Set initial system information in RAM */
 #if BIOS_ROM_HARDWARE != 0
         move.b  #0x80, BIOS_MVS_FLAG
+        /* Init credits information, and assume credits
+         * are always stored in P1_CREDITS_BCD
+        */
+        move.b  #0, BIOS_STATCURNT
+        move.b  #0, BIOS_STATCURNT_RAW
         move.b  #0, P1_CREDITS_BCD
         jsr     load_soft_dip
 #else
@@ -208,7 +213,17 @@ NO_OP:
 
 
 SYSTEM_IO:
-        /* player 1 */
+        /* check player 1 coin button */
+        move.b  BIOS_P1CURRENT, %d1
+        move.b  %d1, BIOS_P1PREVIOUS
+        move.b  REG_P1CNT, %d0
+        not.b   %d0
+        move.b  %d0, BIOS_P1CURRENT
+        eor.b   %d0, %d1
+        and.b   %d1, %d0
+        move.b  %d0, BIOS_P1CHANGE
+
+        /* check player 2 coin button */
         move.b  BIOS_P1CURRENT, %d1
         move.b  %d1, BIOS_P1PREVIOUS
         move.b  REG_P1CNT, %d0
@@ -273,11 +288,18 @@ SYSTEM_IO:
 
 .Lstart_status:
         /* start status */
-        move.b  BIOS_STATCURNT, %d1
-        move.b  %d1, BIOS_STATCURNT_RAW
         move.b  REG_STATUS_B, %d0
         not.b   %d0
-        andi.b  #0xf, %d0
+        /* bits 1 and 3 from reg_status_b (start buttons)
+         * to bit 1 and 2 in %d0 (for bios_start_flag)
+         */
+        move.b  %d0, %d1
+        andi.b  #0x4, %d1
+        lsr     #1, %d1
+        andi.b  #0x1, %d0
+        or.b    %d1, %d0
+        move.b  BIOS_STATCURNT, %d1
+        move.b  %d1, BIOS_STATCURNT_RAW
         move.b  %d0, BIOS_STATCURNT
         eor.b   %d0, %d1
         and.b   %d1, %d0
@@ -286,12 +308,30 @@ SYSTEM_IO:
         beq     .Lnostart
 .Lstart_pressed:
         /* when start is pressed and credits are available:
-         *   . game's "plater_start" callback is executed
+         *   . game's "player_start" callback is executed
+         *   . the callback sees which player wants to start
+         *     via bits set in bios_start_flag
          *   . the callback engages "game mode" by
          *     setting bios_user_mode set to 2.
          * [only when game mode is engaged]
-         *   . credits are decremented
+         *   . credits are decremented for each bit still set
+         *     in bios_start_flag when the callback exits
+         * [before the game is started]
+         *   . hitting 2UP start means that two players want
+         *     to start, and it only runs the callback if
+         *     enough credits are available
          */
+        cmp.b   #2, BIOS_USER_MODE
+        beq     .Lend_of_coin_prechecks
+        btst    #1, %d0
+        beq     .L2up_no_pressed
+        bset    #0, %d0
+        cmp.b   #2, P1_CREDITS_BCD
+        blt     .Lnostart
+.L2up_no_pressed:
+        cmp.b   #1, P1_CREDITS_BCD
+        blt     .Lnostart
+.Lend_of_coin_prechecks:
         move.b  %d0, BIOS_START_FLAG
         jsr     PLAYER_START.l
         cmp.b   #2, BIOS_USER_MODE